1. Field of the Invention
The present invention relates to a technology for adjusting curvature and inclination of the scanning lens in an optical scanning apparatus and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus employed in a laser beam printer, digital copier, or laser facsimile machine includes a regular polygon mirror that performs deflective scanning of a light beam emitted by a light source, and an optical element (scanning lens) that images the light beam scanned by the polygon mirror on a photosensitive member. Additionally, the image forming apparatus includes a photodetector that determines the starting point of recording. The photodetector is provided on the side where scanning starts, outside an effective exposure area.
In such a color image forming apparatus, a laser beam scanning line curves/inclines depending on curvature characteristics of the optical element, skew of the optical housing, heat distortion of a motor driving the polygon mirror, heat distortion within the main unit due to heat for another unit, skew in the photosensitive member, etc. Due to the curvature/inclination in the scanning line, three or four scanning lines fail to merge, resulting in color shift.
To counter this problem, Japanese Patent Laid-Open Publication No. H11-287966 discloses a technology in which a plate-shaped glass is provided inside the optical housing along the scanning direction. The curvature/inclination of the scanning line can be adjusted by placing the glass in the housing tilted in cross-sectional view or by changing the thickness of the glass.
However, even though the amount of curvature of the scanning line can be adjusted by this conventional technology, the inclination of the scanning line cannot be adjusted. Therefore, the inability to adjust the inclination of the scanning line leads to degradation of image quality due to unevenness of color, color shift, and the like. Further, inserting the glass in the housing degrades the optical characteristics by causing wavefront aberration, etc.
Japanese Patent Laid-Open Publication No. 2001-100135 discloses a technology in which the inclination of the scanning line is adjusted by inclining a folded mirror in the optical housing perpendicular to the optical axis.
However, in this conventional technology, even though the inclination of the scanning line can be adjusted, adjustment of the inclination results in variation of the curvature of the scanning line, necessitating further readjustments and making it difficult to meet the correction values. Further, after correction of the curvature/inclination of the scanning line, the magnification of each toner image varies (the light path length from each toner image varies because of the spinning of the folded mirror).
To correct the varying magnification of the different toner images, a technology is disclosed in Japanese Patent Laid-Open Publication No. 2002-182145, in which a mechanism for adjusting the curvature/inclination of the scanning line is provided in a long lens corresponding to each semiconductor laser.
However, the conventional technology disclosed in Japanese Patent Laid-Open Publication No. 2002-182145 described above present the following problem.
Though the mechanism provided for adjusting the curvature/inclination of the scanning line takes care of the problem faced in the earlier conventional technology, the mechanism itself takes up a large space and is difficult to be placed inside the optical housing. In addition, the mechanism pushes up the cost significantly.